Postal Vita is a port of Postal, a classic shooter game made by Running With Scissors in 1997.
The port supports audio, widescreen and twinsticks.

Directly from Steam:
Take control of The Postal Dude in his infamous first outing as he battles to survive in a world gone mad! POSTAL is a classic isometric shooter filled to the brim with fast-paced explosive action. Crazed gunmen out for your blood await you around every corner, and your only choice is to get them before they get you. Fight back with a destructive arsenal as make your way through a violence-stricken town.

Installation:

Install the .vpk file and extract data files archive in ux0:/data.
You'll also need game files from Postal 1 installation (res and title folders). Place those two folders in ux0:/data/Postal.
Postal game files can be bought on GOG or Steam for 1,99€ at the time of writing.